1. Field of the Invention
The present invention relates to speed reducing gear technology, and more particularly, to a harmonic drive that improves transmission accuracy.
2. Description of the Related Art
Harmonic drive is a high ratio of gear reducer. A conventional harmonic drive generally comprises a rigid internal gear, a flexible external gear rotatably mounted within the rigid internal gear, and a wave generator rotatably mounted within the flexible external gear. After installation of the wave generator in the flexible external gear, the flexible external gear is pushed by the outer perimeter of the wave generator and elastically deformed to provide an elliptical shape. Thus, when the wave generator is driven to rotate by a power source, the rigid internal gear and the flexible external gear are forced to mesh with each other in the major axis of the wave generator and to disengage from each other in the minor axis of the wave generator. Due to a difference in the number of teeth between the rigid internal gear and the flexible external gear, a high speed reduction ratio will be achieved to provide a high torque output when the wave generator is been continuously rotated.
However, because the flexible external gear will become oval in shape after it is inserted into the wave generator, a fretting wear problem will occur during power transmission through the inner perimeter of the flexible external gear, and the transmission accuracy will be affected after a long use. In order to eliminate this problem, Japan Utility Model Pub. No. 6-19872 teaches a measure of processing a recessed portion out of the outer peripheral edge of the axle bearing of the wave generator, wherein the recessed portion and a width of the flexible external gear exhibit a predetermined ratio so that the thrust force produced during installation of the flexible external gear can be reduced, thereby reducing fretting wear. However, in actual application, the effect of improvement of this measure is limited.